Conventionally, the wheel mountable units of a tire pressure monitoring system (TPMS) include a motion sensing device for detecting when the wheel on which they are mounted is rolling, and thus when the vehicle is moving. The TPMS may therefore adopt a “sleep” mode to conserve battery energy when the vehicle is stationary. Typically the motion sensing devices are either accelerometers or shock sensors. Shock sensors have the disadvantage that they are large and occupy a significant portion of the limited space available on wheel unit, typically on its printed circuit board (PCB). They also require an interface circuit that has a high input impedance. The interface circuit is sensitive to any contaminates on the PCB such as flux residue and moisture. These issues can lead to false detection of motion when the vehicle is not moving; this problem is sometimes known as constant wake. Eliminating the shock sensor would remove these problems and also reduce cost.
Accelerometers are generally packaged with at least a signal conditioning circuit and/or an ASIC or data processing integrated circuit, and are typically micro-machined using established MEMS processes. Accelerometers have the disadvantages of requiring calibration and being expensive.
It would be desirable therefore to provide alternative means for detecting wheel rotation and therefore vehicle motion.